The highest web speeds in paper machines are currently of an order of about 25 meters per second, but before long, a speed range of from about 25 m/s to about 40 m/s is likely to be taken into use. Even with the highest running speeds that are employed now, and with the ever higher running speeds in the future, the dryer section has become and will likely remain a bottle-neck for the runnability of a paper machine.
In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders comprise two wires which press the web, one from above and the other one from below, against heated faces of drying cylinders. Between the rows of cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering which may result in web breaks. In single-wire draw, each group of drying cylinders comprises only one drying wire on whose support the web runs through the entire group so that, on the drying cylinders, the drying wire presses the web against the heated cylinder faces, and on the reversing cylinders situated between the drying cylinders, the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop, and the reversing cylinders are placed inside the wire loop. In prior art "normal" groups with single-wire draw, the heated drying cylinders are placed in the upper row, and the reversing cylinders are placed in the lower row, these rows generally being horizontal and parallel to one another. So-called "inverted" groups with single-wire draw are also known, in which the heated drying cylinders are placed in the lower row and the reversing suction cylinders or rolls in the upper row, the substantial objective of such inverted groups being to dry the web from the side opposite in relation to the side of the web dried in a normal group with single-wire draw.
In the area of the dryer section of a paper machine, various problems have occurred, for which the present invention suggests novel efficient solutions. These problems include the large length of the dryer section which increases the costs of the dryer section and the machine hall. It is not desirable to enlarge the diameter of drying cylinders in order to increase the capacity because the drying cylinder is basically a pressure vessel and large rotating masses create problems. Problems have also been caused by the difference in speed between the paper web and the wires, which has resulted in wear of the wires and, at the worst, even in paper breaks in the dryer section. Problems also have occurred in the controllability of the web draw and in the runnability of the web. The cross-direction shrinkage which deteriorates the quality of the paper or board, has also been a problem, especially when the cross-direction shrinkage is uneven.
With respect to the prior art related to the present invention, reference is made, for example, to the following publications.
Swedish Patent No. 463,568 (corresponding to International Publication No. 90/14467) describes a method for drying paper in a paper machine by means of which cross-direction shrinkage of the paper web is influenced and favorably prevented in a paper machine comprising at least one heated drying cylinder and at least one drying wire. In the paper machine, the paper web is passed over the drying cylinder in direct contact with its cylinder face at the same time as the drying wire is passed from outside onto the paper web, wherein the longitudinal edges of the paper web are drawn or sucked into contact with one or more cylinders while the web is carried around these cylinders. Thus, in the construction described in this patent, suction is used in connection with the drying cylinder, and the web to be dried is in a direct contact with the face of the drying cylinder. Also, the paper web is supported by the cylinder face over its entire width while the paper web is running over the drying cylinders.
European Patent No. 0 238 470 describes a device in the dryer section of a paper machine, preferably in the dryer section of a cylinder dryer, which device permits control of shrinkage and/or stretch of the paper web in the cross direction of the web in relation to the running direction of the dryer section. The device includes belts, most commonly two belts, which extend or run through the whole dryer section or a part of it and which belts are arranged in the lateral area of the paper web so that they distribute the force that is directed perpendicularly to the running direction of the web and mainly acts in the lateral areas of the paper web. The belt and/or the paper web is coated with an adhesive layer from the side of the web/belt that is placed towards the paper web/belt, in which case, the belt and the paper web act upon each other. In the arrangement described in EP '470, the paper web is attached by its edges to a separate support belt, which support belt is aligned with a groove arranged on the circumference of the cylinder.
Swedish Patent No. 468,217 describes a carrying device for passing the stock web through the dryer section of a paper machine. In the device, carrying belts are arranged on both sides of the stock web in its edge areas in the longitudinal direction, which carrying belts are passed onto rolls, operated by guide members, and grasp corresponding grooves arranged on the rolls, so that shrinkage in the cross direction is prevented. The guide members of each dryer belt comprise a number of individual guides that are placed in pairs above and below the carrier belt and the stock web. In an arrangement in accordance with SE '217, paper is attached from its edges by means of two mechanical chains, and this arrangement is meant for very slow running speeds only.
Finnish Patent Application No. 895928 (corresponding to U.S. Pat. No. 5,135,614) describes a suction roll in which the paper web adheres to the face of the suction roll over its whole width, and an intensified hold is arranged in the lateral areas of the suction roll. The suction roll comprises a perforated roll mantle and a suction space inside the roll mantle which can be subjected to a vacuum. A suction flow thus enters through the perforations in the roll into the roll interior and the paper web is pressed towards the outer face of the roll mantle. The suction space is divided at least into three vacuum spaces in the direction of width of the roll, while the suction space comprises at least two partition walls inside the roll. By means of the partition walls, the suction roll is divided into different vacuum zones such that it is possible to provide the outer vacuum spaces with a higher vacuum than the vacuum space in the mid area of the roll, whereby the vacuum profile is arranged to be growing towards the edges of the roll across the width of the roll and the percentage of shrinkage in the lateral areas of the paper web is reduced, which has a favorable effect on the evenness of the shrinkage.
Finnish Patent No. 84088 (corresponding to U.S. Pat. No. 5,397,438) describes a method in the transfer of a paper web for reducing and equalizing the cross-direction shrinkage of the paper web in the dryer section of a paper machine. The drying wire is provided, in its lateral areas, with an adhesive substance for the time of the process, in which connection, by means of adhesion means, an adhesion force is produced in the drying stage between the lateral areas of the wire and the paper, whereby cross-direction shrinkage of the paper web is prevented. The adhesive substance is removed when it is no longer needed.
U.S. Pat. No. 4,980,979 describes a suction roll whose function is to provide at least one end of the roll with a higher vacuum level than the rest of the roll in order to make threading of the web easier and which suction roll thus has a function corresponding to that described in Finnish Patent Application No. 895928.
With further respect to the prior art, reference is made to Finnish Patents Nos. 64,335 and 82,019 (corresponding to U.S. Pat. No. 5,199,623) which describe arrangements in which a support wire is not used, but the web is carried by an airborne nozzle blowing.
With respect to additional prior art closely related to the present invention, reference is made to the current assignee's Finnish Patent Application No. 943040 (corresponding to U.S. Pat. No. 5,575,084) which describes a method and device for drying and cooling a paper web or equivalent. In the method, for drying and cooling the paper web or equivalent, the web is passed over the circumference of a revolving roll or equivalent on support of a support wire or equivalent on the face of the support wire or equivalent that is placed facing the roll, and the web is dried and/or cooled by means of a gas. It has been considered one novelty of this method that drying and/or cooling gas is blown through openings formed in the mantle of the roll into the space between the outer face of the roll and the web supported by the support wire or equivalent, whereby a support zone formed by pressurized gas is formed between the outer face of the roll and the web. Moistened or humidified gas is passed out of the support zone into the interior of the roll through exhaust openings formed in the mantle of the roll, and more specifically into a system of ducts placed inside the roll.